Toyota 2013 Prius Owner's Manual page 65

1-1. Plug-in hybrid system
Charging (→P. 80)
Vehicle proximity notification system
When driving with the gasoline engine stopped, a sound, which
changes in accordance with the driving speed, will be played in order
to warn people nearby of the vehicle's approach. The sound will stop
when the vehicle speed exceeds approximately 15 mph (25 km/h).
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PRIUS PHV_OM_OM47787U_(U)
In order to drive in EV mode,
charge the hybrid battery (trac-
tion battery) from an external
power source before using your
vehicle.
If there is not enough charge
remaining in the hybrid battery
(traction battery), the EV driving
range will become shorter.